On Arizona Milkweed. Butterfly garden, clearing in Madrean Woodland. Not interacting with the lycid beetles; just happened to land there. Somewhat larger than (other?) Prionyx on same plant. Those had dark maculation on dorsum of abdomen. Unlike those smaller Prionyx, which are generally present, but in small numbers, this larger, brighter red, and unmaculated individual was seen only once in several hours of searching at four sites.